In order to understand some of the functions of neurons in the basal forebrain in magnocellular complex (BFMC), it is essential to identify the sources and transmitter specificities of afferents to this system. Using axonal tracing and transmitter immunocytochemistry (ICC), Pilot 4 is designed to characterize, at the light and ultrastructural levels, neural systems that innervate BFMC cholinergic neurons in rodents. The anterograde tracer Phaseolus vulagaris leukoagglutinin (PHA-L) will be iontophoresed into regions known to be labeled following injections of retrograde tracer into subdivisions of the BFMC, and dual-color ICC will be employed to identify specific interactions between PHA-L-labeled boutons and cholinergic neurons. Monoaminergic, cholinergic, gamma- aminobutyric acid (GABA)ergic, and peptidergic transmitter-specific interactions with cholinergic neurons of the BFMC will be identified at the electron-microscopic level using double-label ICC> These studies will provide new insights concerning afferents that modulate cholinergic BFMC neurons in rodents and are essential for similar investigations of these afferents in primates.